In optical communications networks, optical transceivers are used to transmit and receive optical signals over optical fibers. An optical transceiver generates amplitude and/or phase and/or polarization modulated optical signals that represent data, which are then transmitted over an optical fiber coupled to the transceiver. Each transceiver includes a transmitter side and a receiver side. On the transmitter side, a laser light source generates laser light and an optical coupling system receives the laser light and optically couples, or images, the light onto an end of an optical fiber. The laser light source typically is made up of one or more laser diodes that generate light of a particular wavelength or wavelength range. The optical coupling system typically includes one or more reflective elements, one or more refractive elements and/or one or more diffractive elements. On the receiver side, a photodiode detects an optical data signal transmitted over an optical fiber and converts the optical data signal into an electrical signal, which is then amplified and processed by electrical circuitry of the receiver side to recover the data. The combination of the optical transceivers connected on each end of the optical fiber and the optical fiber itself is commonly referred to as an optical fiber link.
In high-speed optical fiber links (e.g., 10 Gigabits per second (Gb/s) and higher), multimode optical fibers are often used to carry the optical data signals. In such links, certain link performance characteristics, such as the link transmission distance, for example, are dependent in part on the design of the optical coupling system, the modal bandwidth of the fiber, and the relative intensity noise (RIN) of the laser diode. The modal bandwidth of the fiber and the RIN of the laser diode can be affected by the launch conditions of the laser light into the end of the multimode optical fiber. The launch conditions are, in turn, dependent upon the properties of the laser diode itself and upon the design and configuration of the optical coupling system. Due to limitations on the manufacturability of optical elements that are typically used in optical coupling systems, the ability to control the launch conditions is limited primarily to designing and configuring the optical coupling system to control the manner in which it images the light from the laser diode onto the end of the fiber.
Due to the nature of the processes that are used to manufacture multimode fibers, center and edge defects often exist in the refractive index profiles of the fibers. The existence of the defects can dramatically change the effective modal bandwidth of the fiber and degrade it below the out-of-factory minimum specification. For these reasons, efforts are often made to control the launch conditions of the laser diode to prevent the laser light from passing through the areas in the fiber where the defects are most severe and where the occurrence of the defects is more frequent. For example, in spiral launch optical coupling systems, the laser light is encoded with a phase pattern that rotates the phase of the light linearly around the optical axis of a collimating lens that is used to couple the light onto the end of the optical fiber. Rotating the phase of the laser light about the optical axis helps to ensure that refractive index defects in the center of the fiber are avoided. In addition, the spiral launch methodology is relatively successful at reducing optical feedback from the fiber end to the laser aperture, which can destabilize the laser.
While various transceiver and optical fiber link designs enable the overall bandwidth, or data rate, of optical fiber links to be increased, there are limitations on the extent to which currently available technologies can be used to improve the bandwidth of an optical fiber link. For example, there is currently a need for multimode optical fiber links that operate at data rates that are well in excess of the data rates at which currently available laser diodes are capable of operating. In particular, a class of multimode optical fibers commonly referred to as OM3 multimode optical fibers are optimized for use with vertical cavity surface emitting laser diodes (VCSELs) that operate near a wavelength of 840 nanometers (nm). Currently, however, no commercially available VCSELs operate at speeds greater than around 10 Gb/s.
It has been shown that receiver-based electronic dispersion compensation (EDC) techniques in combination with particular modulation formats can be used to provide optical fiber links that operate at speeds that are higher than the speeds at which the VCSELs of the link transceivers operate. Such links, however, are generally limited to data rates of about 20 Gb/s.
It is also known that multiple optical links can be combined to achieve an optical link having a higher data rate than that of each of the individual optical links that form the combination. For example, it is known to combine four 10 Gb/s optical links to achieve an optical link having an overall bandwidth of 40 Gb/s optical links. However, in order to achieve such a link, four sets of parallel optics and four optical fibers are needed, which significantly adds to the costs associated with such links. The number of sets of parallel optics and optical fibers that would be needed to form such a link could be reduced by using different wavelengths for the optical signals that are being transmitted and received. Using different wavelengths for the optical signals being transmitted and received, however, would require the use of laser diodes that transmit light at different wavelengths, which can increase the costs associated with the optical link. In addition, using different wavelengths over the same optical paths would require the use of wavelength division multiplexing (WDM) to separate the optical data signals.
A need exists for a bi-directional optical link that uses a single multimode optical fiber and a single wavelength and that is capable of operating at data rates that are in excess of 20 Gb/s. A need also exists for a bi-directional duplex optical link that is capable of operating at data rates in excess of 40 Gb/s and which does not require a set of parallel optics and fibers for each optical path.